1. Field of the Invention
The present invention relates generally to a transmission method and transmission apparatus for transmitting data of various formats on a bus conforming to the IEEE 1394 standard.
2. Description of the Related Art
Networks using an IEEE 1394 bus can be used to connect a plurality of devices to transmit video data, audio data, and other types of data between the devices. IEEE 1394 standard buses are designed to use an isochronous transmission channel for transmitting large capacity data such as video and audio data, and an asynchronous transmission channel for transmitting control commands. Isochronous and asynchronous data are time division multiplexed on the bus. Details of the protocol for transmitting audio data (music data) on an IEEE 1394 standard bus are disclosed in xe2x80x9cAudio and Music Data Transmission Protocol,xe2x80x9d IEC-PAS Publication 61883-6 (ed. 1.0 1998). This transmission protocol proscribes a label section at the head of each data packet to indicate the format of the data which follows.
However, this audio data transmission protocol only covers 16 and 24 bit format audio data with a fixed sampling frequency of 44.1 kHz. Numerous other audio data formats have been developed to further improve the sound quality. For example, several multi-channel digital audio formats have recently been proposed. Thus, there are a large number of formats for digital audio data transmission across an IEEE 1394 bus that are not addressed by the aforementioned transmission protocol. Furthermore, because there are so many different data formats and new formats are constantly being developed, not all devices can accept all of the available data formats.
In situations where audio data to be transmitted via an IEEE 1394 bus is absent (e.g., missing, incomplete, intentionally blank, or not ready to be transmitted), the present transmission protocol simply indicates the absence of data by placing an Ext-No DATA indicator in the label section of the data packet. In this situation, the label section does not indicate the format of the missing data. Hence, this protocol does not allow the receiving side to determine the format of the missing data. Knowing the format of the absent data is useful in detecting/anticipating changes between data formats (i.e., several formats may be transmitted in a time-division manner) and whether the various devices on the bus are compatible with the format.
Heretofore, only audio data formats have been discussed, but this problem applies equally to other types of data (e.g., video, text, pictures, etc . . . ).
It is therefore an object of the present invention to provide information to a data receiver for determining the format of data during a data absence state.
Other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification and the drawings.
To achieve the above-mentioned object, a transmission method is presented for transmitting formatted data in predetermined length units between devices connected to a bus, in which when the data to be transmitted is not present, the transmission method includes the steps of disposing label data representing a data absence in a first section of the predetermined length unit; disposing data representing the format of the absent data, in a second section of the predetermined length unit; and transmitting the predetermined length unit. According to this transmission method, the format of the absent data can be distinguished on the basis of the data disposed in the second section.
A transmission apparatus according to the invention includes a data input for obtaining data; a data generator for dividing the data obtained by the data input means into units having a predetermined data length, disposing label data representing a scheme of the data in a head portion of each unit resulting from the division, the data generator disposing, in response to an absence of data input to the data input, label data representing the data absence and data representing the format of the absent data in predetermined sections, respectively; and a transmitter for sending out the data generated by the data generator onto a bus.
When desired data is not present, data transmitted from this transmission apparatus contains data representing the format of the data which is not present. According to a first aspect of a transmission method of the present invention, the format of the data which is not present is known from the data disposed in the second section. Thus, the receiving side knows what format data is not present. For example, the receiving side knows whether the absent data is audio data of such a format that the receiver can process data of that format. Then, when data to be transmitted is present, the receiving side is prepared to receive data of that format.
According to a second aspect of the present invention, data representing the format of the absent data is obtained by selecting a specific value from a plurality of values defined as label data for that format. As a result, the receiving side need only read the specific value to determine the format of the absent data.
According to a third aspect of the present invention, random data is disposed in a third section of the transmission data unit. Even if, for example, data absence states are consecutive, therefore, it becomes possible to disperse encrypted and transmitted data at random. As a result, the safety of the data transmission can be improved.